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Tutor. To show you the nature of conducting-bodies, I will now throw another brass chain over the conductor, so that one end of it may lie on the floor. See now if you can get any sparks while I turn the machine.

James. No, I can get none, put my kuckle as near to it as I will.-Does it all run away by the chain ?

Tutor. It does ; a piece of brass or iron wire would do as well ; and so would any conducting substance which touched the conductor with one end, and the floor with the other: your body would do as well as the chain. Place your hand on the conductor, while I turn round the cylinder: and let your brother bring his knuckle near the conductor.

Charles I can get no spark.

Tutor. It runs through James to the earth, and you see his body is a conductor as well as the chain. With a very little contrivance, I can take sparks from you or James, as well as you did from the conductor.

James. I should like to see how that is done.

Tutor. Here is a small stool, having a mahogany top and glass legs. If you stand on that, and put your hand on the conductor, the electricity will pass from the conductor to your body.

Charles. Will the glass legs prevent it from running from him to the earth?

Tutor. They will: and therefore what he receives from the conductor, he will be ready to part with to any of the surrounding bodies, or to you if you bring your hand near enough to any part of him.

James. The sparks are more painful in coming through my clothes, than when I received them on my bare hand.

Tutor. They are : you understand, I hope, the process.

Charles. By means of the chain trailing on the ground, the electric duid is collected from the earth on the glass cylinder, which gives it through the points to the conductor: from this it may be conveyed away again by means of other conductors.

Tutor. Whatever body is supported, or prevented from touching the earth, or communicating with it, by means of glass or other non-conducting substances, is said to be insulated. Thus a body suspended on a silk line is insulated, and so is any substance that stands on glass, or resin, or wax, provided that these are in a dry state, for moisture will conduct away the electric fluid from any charged body.

CONVERSATION XXX.

Of the Electrical Machine.

CHARLES. What is that shining stuff which I saw you put to the rubber yesterday?

Tutor. It is called amalgam : the rubber, by itself, would produce but a slight excitation : its power, however, is greatly increased by laying upon it a little of this amalgam, which is made of quicksilver, zinc, and tinfoil, with a little tallow or mutton suet.

James. Is their any art required in using this amalgam ?

Tutor. When the rubber and silk flap are very clean and dry, and in their place, then spread a little of the amalgam upon a piece

of leather, and apply it to the upper part of the glass cylinder, while it is revolving from you ; by this means, particles of the amalgam will be carried by the glass itself to the lower part of the rubber, and will increase the excitation.

Charles. I think I once saw a globe, instead of a cylinder, for an electrical machine.

Tutor. You might: globes were used before cylinders, but the latter are the most convenient of the two. The most powerful electrical machines are fitted with flat plates of glass. In our experiments we shall be content with the cylinder, which will answer every purpose of explaining the principles of the science.

James. As I was able to conduct the electricity from the tin conductor to the ground, could I likewise act the part of the chain, by conducting the fluid from the earth to the cushion ?

Futor. Undoubtedly: I will take off the chain, and now do you keep your hand on the cushion while I turn the handle.

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